Nylon is the name given to a family of polyamide polymers characterized by the presence of the amide group --CONH-- as a recurring unit of the main chain of the polymer. Notably, this polymer is seen as very weak structurally because the amide groups in nylon can be cleaved by active species such as free radicals or other oxidative reagents. Nylons are formed by reacting diacids and diamines through polycondensation to form a variety of nylons such as 66, 6, 610, 46 etc.
Thus, in order to inhibit the oxidative degradation of these nylons, additives such as antioxidants have been utilized. Examples of suitable antioxidants useful in improving nylon stability include certain alkylated phenols, polyphenolic compounds, aromatic amines and organic phosphites. Notably, the incorporation of an antioxidant or similar additive into nylon has been shown to provide improved resistance to both thermal degradation and photo oxidation based on the principle that the antioxidant destroys the oxidative species, thus preventing the dissociation of nylon molecules caused by those oxidative species.
In fact, numerous patents have been issued relating to the incorporation of an antioxidant into nylon for use as a stabilizer. For example, Thompson U.S. Pat. No. 3,180,849 adds phenyl halophosphine as an antioxidant stabilizing additive to nylon, while Rothrock U.S. Pat. No. 2,493,597 adds phosphite esters. Furthermore, Stamatoff U.S. Pat. No. 2,630,421 describes the synergistic combination of aromatic amines and phenols for resistance to thermal oxidation of nylon. Other organic stabilizers reported to be effective against nylon oxidation include stable radicals such as nitroxides and hydrazyls as described in Tazewell U.S. Pat. No. 3,477,986. Still further, Stokes U.S. Pat. No. 3,454,412 describes the mixing of UV absorbers and antioxidant for low melting nylon.
Although improving the thermal and light stability of nylon will extend its useful service life in many applications, several other applications require that nylon be stable in the presence of steam and/or hot water. In view of the fact that nylon has always been regarded as subject to degradation by hot water, improved hydrolysis resistant grades of nylon have long been sought.
At present, so-called hydrolysis resistant grades of nylon with improved performance in hot water use are commercially available for thermally molded products. These improved grades, however, are essentially nylons with antioxidant additives incorporated therein, and therefore, are essentially antioxidative grades of nylon. One example of such a nylon is the antioxidative grade Zytel 122L, commercially available form E. I. Du Pont de Nemours Co., Wilmington, Del. It has been shown that Zytel 122L has triple the service life in a hot water environment at 77.degree. C. as compared to the non-additive grade of nylon Zytel 101, also commercially available from E. I. Du Pont de Nemours Co.
However, it has been clearly shown that the changes which take place for the molecular dissociation in nylon may not be simply related to hot water, but rather to the combination of hot water and oxygen dissolved in water. Thus, the oxygen content in water is inversely related to the useful life of nylon. For example, stagnant hot water in which oxygen is used up and is not replaced causes fewer changes to nylon than flowing hot water in which oxygen is continuously available. Also, as would be expected, the changes are also temperature dependent.
Nylon, such as Zytel 122L, which uses antioxidants or similar additives to inhibit hydrolytic degradation has been utilized extensively for injection or extrusion molding resins to produce a wide variety of plastic parts. However, this type of antioxidant-containing nylon does not inhibit hydrolytic degradation upon the formation of a membrane.
Nylon membranes are produced by dissolving nylon and minor ingredients in a solvent such as formic acid to form a polymeric dope solution. The dope is formed into a thin layer and quenched in a bath containing a non-solvent system such as methanol and/or water, whereupon micropores are formed in the resulting polymeric membrane. For a more detailed description of the process for preparing skinless microporous nylon membranes in which formic acid is used as a solvent to dissolve nylon, see Marinaccio et al. U.S. Pat. No. 3,876,738, Knight U.S. Pat. No. 5,084,179, and Pall U.S. Pat. No. 4,340,479, all of which are hereby incorporated by reference.
Unfortunately nylon membranes produced by the method(s) described hereinabove have amide groups which are susceptible to attack by oxidative species such as free radicals, and therefore, are not sufficiently resistant to hydrolytic degradation for many applications. Such applications would include, for example, filtration in the food and beverage industry, pharmaceuticals, electronics and other applications where hydrolytic stability is required. Desirably, filter elements for filter cartridges can be made from hydrolytically stable nylon membranes, with the membrane being cast on a support web. These cartridges should preferably withstand repeated cycling totalling at least 20 hours autoclaving with saturated steam at about 120.degree. C. and/or at least 200 hours hot water flushing at about 80.degree. C. in order to be suitable for use in the hereinabove cited applications.
Even when nylon having an antioxidant added thereto is employed as the raw material for the nylon membrane, there is still no improved resistance to hydrolytic degradation. Thus, the antioxidative effect found in pellets of nylon is missing when the nylon is formed into a membrane. It is believed that the antioxidant in the nylon is either insoluble in nylon solvent (formic acid) or its antioxidative effect is rendered ineffective or at least loses its chemical reactivity in the presence of other compounds of the polymer solution. Application of antioxidant to the membrane after membrane formation can be effective, but requires that the antioxidant so applied either be approved for its intended end-use, as for example Code of Federal Regulations approval for food contact, or be virtually non-extractable for critical applications such as are found in the pharmaceutical and electronics industries.
The need exists for a hydrolytically stable nylon membrane which is capable of withstanding repeated autoclaving and hot water sanitization without molecular dissociation for applications cited hereinabove. This invention describes a method for the preparation of these membranes.